In general, refrigerators such as compression-type refrigerators comprising a compressor, a condenser, an expansion valve and an evaporator have a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. Heretofore, chloro-fluorocarbons such as dichlorodifluoromethane (R-12)-and chlorodifluoro-methane (R-22) have been used as the refrigerant for the compression-type refrigerators and various types of lubricating oils have been produced and used in combination with the refrigerant. However, since there is the anxiety that the chlorofluorocarbons cause environmental pollution such as ozonosphere destruction in stratosphere when these substances are released into the atmosphere, the regulation on the chlorofluoro-carbons is becoming stricter worldwide. Due to this situation, novel refrigerants such as hydrofluorocarbons and fluorocarbons, typical examples of which include 1,1,1,2-tetrafluoroethane (R-134a), are attracting attention. Although there is no anxiety that the hydrofluorocarbons and the fluorocarbons destruct the ozonosphere, there is the fear that these substances might cause global warming due to the longevity of these substances in the atmosphere. Therefore, the use of natural substance-based refrigerants that do not cause the above problems is considered.
Among the natural substance-based refrigerants, ammonia has heretofore been used for industrial refrigerators that require oil agents and refrigerants in great amounts. As the refrigerating oil for such refrigerators, mineral oils have been used. However, since mineral oils and ammonia are not miscible with each other, additional apparatuses such as an apparatus for separation of the oil are indispensable. Therefore, the system becomes great and, moreover, the performance of the system is not sufficiently satisfactory.
In Japanese Patent Application Laid-Open No. Heisei 5(1993)-9483, it is disclosed that polyalkylene glycols which do not have hydroxyl group at any of the ends have excellent miscibility with ammonia and contribute to improvement in the performance to a great extent. It is described in the specification of the above application that additives conventionally used for refrigerating oils such as extreme pressure agents and antioxidants can be used. However, the use of the additives is not shown in the examples.
In Japanese Patent Application Laid-Open No. Heisei 6(1994)-10081, additives advantageously used in combination with ammonia or hydrofluorocarbon refrigerants are disclosed. However, the use of the additives specifically described in the examples is limited to the use in combination with hydrofluorocarbon refrigerants.
In general, since ammonia has a greater reactivity than other refrigerants and exhibits basic property by itself, acidic substances have been considered to be unsuitable for use in combination with ammonia. On the other hand, most of the additives are actually acidic substances. This situation causes insufficient lubrication and the improvement has been desired. The present situation described above is reflected on the fact that the above references have no specific descriptions on the use of combinations of ammonia with conventional additives. It has not been found whether the combinations can be used for practical applications.
The present invention has an object of providing a refrigerating oil composition having excellent miscibility with natural substance-based refrigerants and, in particular, with ammonia-based refrigerants and exhibiting an improved lubricity.
As the result of extensive studies by the present inventors, it was found that the object of the present invention could be effectively achieved when the refrigerating oil composition comprises a synthetic oil component comprising a polyether compound having a specific property and a mineral oil component comprising sulfur in specific relative amounts, and the amount of the sulfur components is in the range of 5 to 1,000 ppm in the composition. The present invention has been completed based on this finding.